helper/shadow: KeyedValue.WaitForChange

helper/shadow/keyed_value.go

@@ -42,6 +42,26 @@ func (w *KeyedValue) Value(k string) interface{} {
 	return val.Value()
 }
 
+// WaitForChange waits for the value with the given key to be set again.
+// If the key isn't set, it'll wait for an initial value. Note that while
+// it is called "WaitForChange", the value isn't guaranteed to _change_;
+// this will return when a SetValue is called for the given k.
+func (w *KeyedValue) WaitForChange(k string) interface{} {
+	w.lock.Lock()
+	w.once.Do(w.init)
+
+	// Check for an active waiter. If there isn't one, make it
+	val := w.waiters[k]
+	if val == nil {
+		val = new(Value)
+		w.waiters[k] = val
+	}
+	w.lock.Unlock()
+
+	// And wait
+	return val.Value()
+}
+
 // ValueOk gets the value for the given key, returning immediately if the
 // value doesn't exist. The second return argument is true if the value exists.
 func (w *KeyedValue) ValueOk(k string) (interface{}, bool) {

helper/shadow/keyed_value_test.go

@@ -168,3 +168,58 @@ func TestKeyedValueClose_existingBlocked(t *testing.T) {
 		t.Fatalf("bad: %#v", val)
 	}
 }
+
+func TestKeyedValueWaitForChange(t *testing.T) {
+	var v KeyedValue
+
+	// Set a value
+	v.SetValue("foo", 42)
+
+	// Start reading this should be blocking
+	valueCh := make(chan interface{})
+	go func() {
+		valueCh <- v.WaitForChange("foo")
+	}()
+
+	// We should not get the value
+	select {
+	case <-valueCh:
+		t.Fatal("shouldn't receive value")
+	case <-time.After(10 * time.Millisecond):
+	}
+
+	// Set a new value
+	v.SetValue("foo", 84)
+
+	// Verify
+	val := <-valueCh
+	if val != 84 {
+		t.Fatalf("bad: %#v", val)
+	}
+}
+
+func TestKeyedValueWaitForChange_initial(t *testing.T) {
+	var v KeyedValue
+
+	// Start reading this should be blocking
+	valueCh := make(chan interface{})
+	go func() {
+		valueCh <- v.WaitForChange("foo")
+	}()
+
+	// We should not get the value
+	select {
+	case <-valueCh:
+		t.Fatal("shouldn't receive value")
+	case <-time.After(10 * time.Millisecond):
+	}
+
+	// Set a new value
+	v.SetValue("foo", 84)
+
+	// Verify
+	val := <-valueCh
+	if val != 84 {
+		t.Fatalf("bad: %#v", val)
+	}
+}